The present invention relates to an image forming apparatus, for example, a copying machine, a printer, a facsimile machine, in particular, an image forming apparatus which transfers a toner image on an image bearing member onto intermediary transfer medium, and transfers the image on intermediary transfer medium onto a piece of transfer medium.
There have been known various image formation systems for a color image forming apparatus, for example, a thermal transfer system, an ink jet system, and the like. Among these systems, an electrophotographic system is superior to the rest in terms of image formation speed, image quality, noise level, and the like aspects.
There are also various electrophotographic systems, for example, a multiple layer development system, a multiple layer transfer system, an intermediary transfer system, and the like. According to a multiple layer development system, a plurality of color images (plurality of toner images of different color) are formed in layers on the peripheral surface of a photosensitive member, and then, the plurality of toner images on the photosensitive member are transferred all at once onto a piece of transfer medium. According to a multiple layer transfer system, a plurality of toner images of different color are consecutively transferred onto a piece of transfer medium as each of them is formed. In comparison, according to an intermediary transfer system, a plurality of toner images of different color are consecutively transferred (first transfer) onto an intermediary transfer medium as each of them is formed, and then, the plurality of toner images of different color are transferred all at once (secondary transfer) onto a piece of transfer medium. Among these electrophotographic systems, an intermediary transfer system enjoys substantial advantages: for example, there is little possibility of color mixing, and various transfer media different in quality, thickness, or the like properties, can be used.
FIG. 9 is a schematic sectional view of an image forming apparatus (full color laser beam printer based on four primary colors) which uses one of the conventional intermediary transfer systems, and depicts the general structure of the image forming apparatus.
As depicted by the drawing, along the peripheral surface of the photosensitive drum 1, a charging apparatus 2, an exposing apparatus 3, a developing apparatus 5, an intermediary transfer belt 18, a photosensitive drum cleaner 16, and a discharge roller 17, are disposed in the rotational direction of the photosensitive drum 1 (direction R1 indicated by an arrow mark), in the listed order. The photosensitive drum 1 is an image bearing member, and the exposing apparatus 3 projects a laser beam L onto the peripheral surface of the photosensitive drum 1.
Here, the image forming process of this image forming apparatus will be briefly described.
First, the photosensitive drum 1 is uniformly charged by the charging apparatus 2, and an electrostatic latent image is formed on the peripheral surface of the photosensitive drum 1 by the laser beam L from the exposing apparatus 3. Each electrostatic latent image is developed by one of the developing devices in the developing apparatus 5: yellow color developing device 5a, cyan color developing device 5b, magenta color developing device 5c, and black color developing device 5d, and transferred (first transfer) onto the intermediary transfer belt 18, in a primary transfer station N1. As a result, a color image constituted of four toner images of different color is created.
The color image on the intermediary transfer belt 18 is transferred (secondary transfer) all at once onto a transfer medium P as a transfer roller 7 for the secondary transfer is pressed upon the intermediary transfer belt 18, with the transfer medium P being pinched between the roller 7 and belt 18. After the secondary transfer, the transfer medium P is conveyed to a fixing apparatus (unillustrated), in which the color image consisting of four toner images of different color is fixed to the surface of the transfer medium P by the application of heat and pressure. Thereafter, the transfer medium P is discharged from the image forming apparatus.
In the past, in order to prevent the problem that after a toner image is transferred onto the intermediary transfer belt 18, the toner particles are scattered from the toner image at the locations where the intermediary transfer belt 18 is bent (where rollers 8, 9, and 10 support intermediary transfer belt 18), the intermediary transfer belt 18 was provided with a surface layer with higher volumetric resistivity, located on the side onto which a toner image is transferred.
However, providing the intermediary transfer belt 18 with the surface layer with higher volumetric resistivity often triggered electrical discharge between the photosensitive drum 1 and the intermediary transfer belt 18 during the primary transfer, negatively affecting the image formation process. As a result, a toner image with traces of electrical discharge as illustrated by FIG. 10 was produced; in other words, the so-called xe2x80x9cimage with shark skin texturexe2x80x9d was produced. The effects of this phenomenon were more conspicuous when a half tone image was produced.
The primary object of the present invention is to provide an image forming apparatus which can prevent the problem that the quality of a toner image is reduced after the toner image is transferred onto the intermediary transfer medium from the image bearing member.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.